Polyamideimide (PAI) polymer resins are incorporated into compositions used for many high-performance industrial coating applications due to their excellent temperature resistance and high strength. The PAI polymer resin compositions are well known in the literature, for example as described in U.S. Pat. No. 3,494,890 and GB 570,858. One of the primary routes to synthesizing polyamideimide polymers in a form that is convenient for the manufacture and formulation of these coating compositions is by reacting diisocyanate, often 4,4′-methylene diphenyldiisocyanate (MDI) with trimellitic anhydride (TMA). In this process, PAI polymers are typically synthesized in polar aprotic solvents such as N-methyl amide compounds including dimethylformamide, dimethylacetamide, N-methylpyrrolidone (NMP), N-ethylpyrrolidone. See for example U.S. Pat. No. 2,421,021, U.S. Pat. No. 3,260,691, U.S. Pat. No. 3,471,444, U.S. Pat. No. 3,518,230, U.S. Pat. No. 3,817,926, and U.S. Pat. No. 3,847,878.
Alternative solvents such as tetrahydrofuran (THF), methylethyl ketone (MEK), gamma butyrl lactone (GBL), or dimethylsulfoxide (DMSO) have drawbacks such as boiling points which are too low for use as reaction solvent, low polymer resin solubility, or poor storage stability, which may change the performance of the polymer in the application for which it is to be used.
U.S. 2012/065296A1 claims the use of gamma-butyrolactone and cyclopentanone to dissolve the PAI polymer resin to form a coating composition. U.S. Pat. No. 4,950,700 and U.S. Pat. No. 5,095,070 recite examples of gamma-butyrolactone with N-methylamide co-solvents and dimethylol ethylene urea as replacement solvents to synthesize PAI resin. However gamma-butyrolactone has neurological properties that make it subject to regulation and unsuitable for general use in formulations. Dimethylol ethylene urea has not had extensive toxicological studies performed and contains an N-methylamide functionality suspected of negative environmental and health impacts. New solvents, such as those described in U.S.20100076223A1, for example 3-methoxy-N, N-dimethylpropionamide, may be too expensive for practical industrial use or have not been fully tested for long term toxicity.
The typical polymer solids level achieved in these synthetic routes is 35-45% which may be diluted further with solvents or diluents depending on the end-use coating application. These diluents for end-use coating compositions can be either those used in the synthesis reaction to form the polymer resin itself, or other applicable and appropriate solvents which allow the polymer resins to dissolve in, and allow useful preparation of, the end coating compositions. Further, the end coating compositions which are used in such industrial applications as cookware, can coating, and wire coating use a combination of other components which require one or more solvents as diluents of both the PAI, which is typically used as a dispersing polymer, as well as other dispersing polymers, such as polyimides, polysulfones, and polyethersulfones, the primary functional components such as fluorocarbon or other polymers, plus processing aids, additives, fillers, pigments, viscosity adjusting agents, and the like. See for example, U.S. Pat. No. 4,014,834, U.S. Pat. No. 4,259,221, and U.S. Pat. No. 7,858,188. Coating compositions for cookware are described in detail in “Fluorinated Coatings and Finishes Handbook: The Definitive User's Guide”, Laurence W. McKeen, William Andrew Publishing, 2007 and are well know to those skilled in the art.
On a practical level, the polymer solvents known in the art, while they may be useful for either manufacturing PAI or as a solvent in the end-use coating compositions containing PAI and other components, are also recognized for having toxicity concerns. Thus it is an advantage to use solvents for polyamideimide polymers with minimal health and safety impact.